Wierd battery issue's...

Here is the full story. Two weeks ago, I took the truck, 2009 Silverado Crew Cab 4x4, when i went fishing. Truck started and ran fine all day. At the end of the day, I drove it down to my friends house which is about 200 yards from my house. When we went to leave that night, the truck was dead, would not start. In the morning, went down, jumped it, and drove it home. The next day, we took it to AutoZone, they checked it and said it was good. They put a load on it and I came back later and they said it checks out as good as a brand new battery (the battery is 8 months old). So, I thought maybe I left something on in the truck. Took the battery home, started the truck, checked everything in the truck to make sure nothing was on, and left it at that. The next day, I checked the voltage to see if the alternator was working correctly, negative lead to the battery, positive to the alternator and it was showing over 15v. I assumed that is correct.

Last week, I drove the truck several times and it worked perfectly. However, on Sunday, we went to go get groceries and I noticed about half way there that the battery gauge was well below the 14V mark. I guess the gauge was on about 12 or 13. As we got closer to home, it was down to about 10 or 11. I set there before turning it off and checked to make sure everything was off and then turned it off. I figured it would not start and just left it. Yesterday, I went out to jump it, tried starting it first, and it started right up with no hesitation.

Can someone tell me what could possibly have caused all of this? I am at a loss and not really wanting to drive it. The truck is out of warranty, but only has around 54k miles on it. Again, the battery is 8 months old, and have had no battery issues before this.

It is possible that your vehicle has the GM MPG program installed. what this does is shut down the alternator at times to improve MPG.

jumping a battery with the jumping vehicle running will sometimes hide a defective starter [solenoid ] .. next time you get a no start put head lights leave door open [interior lamps] on , crank engine and see if the lights dim. if the lights do not dim then check all connections at the starter . if it repeats then the starter is most likely the problem.

using a voltmeter check for the charging voltage and see if you have the proper voltage across the battery when it will not crank.

cold engine the charge voltage should be around 14.75 volts hot engine the voltage drops to about 13.75volts if its 120 deg f out it may drop to 13.25 volts.

Your truck may have GM's Stand alone voltage regulation. If so there will be a large plastic box on the negative cable. My 2005 has a new battery, a new alternator, and all grounds have been cleaned. When I am driving around the city my voltage gauge reads above 14 volts. When I get on the freeway it drops to around 12-13 volts. When I first noticed the fluctuation it made me very nervous. I did some research and found out that GM designed the system so that the alternator output is computer controlled. You can not even test the alternator out of the truck or it will fry the internals. It is designed to charge the battery to 80% which GM says is optimal for longer battery life. When it hits the 80% and major current draws are not present it will allow the voltage to drop to a point where it is basically just the right amount of output to feed the trucks needs to keep it running. I can drive down the freeway at 70mph with the dvd player, radio, and other lights on and it will stay at 12 volts. If I increase the load, lets say put all 4 windows up and hold the switches after they are all the way up, the voltage will drop for 1/2 a second and then raise up to 14 or so until I release the window switches.

Pikey, thanks for the info, i will look for the black box. I really have never paid attention to the volts before, but since the battery has died once and then did this voltage drop, I have been watching it. Guess I should have been paying more attention. Well, we'll see, need to know if that black box is there first.

09 trucks got the Stand Alone voltage equipment not sure if it was all of them or certain models though. As posted be careful checking the system as you can inadveryantly zap electrical internals.
I see you bought the truck as a repo so chances of owners manuals being with the truck are low but if you do have an owners manual check the electrical section it explains the system somewhat.
We had a post last fall with a member that had problems somewhat like yours dont remember the out come but I do remember many people were surprised to discover the charging system fluctuated so much in new trucks and was computer control.